In broadband systems for transmitting or for retrieving information a synchronization with a useful signal is mostly required.
In such cases a receiver (e.g. a measurement device) often does not have any information a priori about whether and to what extent a useful signal is contained in a received signal or in an input signal. The situation is also rendered more difficult in that the received signal can be distorted or generally changed by a disturbance.
Disturbances often occur as white noise (or Gaussian noise), in which a spectral noise power density is practically constant in a specific more or less large frequency band. Particularly problematic are disturbances which differ from this type of white noise and exhibit pronounced time-selective or frequency selective properties.
Thus is a number of frequency channels can be disturbed over a short period in a receiver or in a measurement device such that a recognition of a useful signal is no longer possible per se in a frequency channel.